Air conditioning systems installed in vehicles in the future will be CO2 air conditioning systems, since, according to EU legislation, the refrigerants R134a, that have been used in vehicle air conditioning systems, will be prohibited by the year 2016.
By contrast to conventional air conditioning systems for vehicles, in which the refrigerant R134a is used, in CO2 air conditioning systems both a pressure signal and a temperature signal are required after the compressor for regulation. The use of two separate sensors to record the pressure signal and to record the temperature signal would involve very great expenditure, and this is why an attempt is made to record the two signals named using a single sensor, a combined pressure and temperature sensor.
Combined pressure and temperature sensors are described, for example, in European Patent No. EP 1 521 061 A1, and German Patent Application Nos. DE 101 09 095 A1 and DE 197 45 244 A1.
In the design approach described in German Patent Application No. DE 101 09 095 A1, a front diaphragm sensor is used having a mounted plunge sensor. However, this concept is very sensitive in the diaphragm area, so that a protective screen has to be inserted. This solution would be designed more for sport racing applications, and therefore involves an extremely short service life.
According to design approach described in German Patent Application No. DE 197 45 244 A1, a separate threaded part is used for the temperature sensor, which is sealed using an O-ring. This concept is rather complex and costly, and is therefore not suitable for mass production. In using an O-ring as sealing element, one should consider that the O-ring, which is an elastomer component, in general tends to become brittle when applied in air conditioning systems, in which lower temperature may definitely be expected, and the sealing effect of the O-ring will deteriorate considerably over the life of the air conditioning system, so that leakages may occur.
German Patent Application No. DE 102 32 315 A1 describes a combined temperature and pressure sensor and a method for ascertaining physical characteristics variables. The combined temperature and pressure sensor described in German Patent Application No. DE 102 32 315 A1 is of the type that is able to be used in fluid systems. In a common base element there is situated at least one channel, the base element being fastened in an opening in a wall, in a pressure-tight manner. In the base element that is developed essentially in a rotationally symmetrical way, a pressure measuring channel is situated straight-through centrically, in the base element, approximately parallel to the pressure measuring channel, at least one temperature measuring channel is situated in an eccentric manner, and the pressure-tight fastening of the base element is developed detachably in the opening of the wall. The end region of the pressure measuring channel, facing away from the wall, runs in a sleeve-shaped elongation of the base element, which is further removed from the wall than the end of the temperature measuring channel facing away from the wall. At the end of the elongation facing away from the wall, there is a pressure sensor having a diaphragm.
A disadvantage of the concepts named above is that the temperature measurement in the combined pressure and temperature sensors introduced there takes place eccentrically. This has the result that the temperature measurement becomes less accurate and in addition, that it depends on the installation position of the temperature sensor.